Throughout human history, supernatural causes have been invoked to explain droughts, earthquakes, thunderstorms, comets, the spread of disease, mental illnesses, mystical experiences, the orbits of the planets, the origin of living things, and the origin of the world, among many other phenomena. As the scientific revolution of the 17th and 18th centuries flourished, appeals to supernatural causation ultimately gave way to successful scientific explanations of various phenomena in terms of natural causes. Ever since its inception, science has increasingly strengthened the plausibility of naturalism by providing informative accounts of a wide range of phenomena in terms of natural causes. The more science has progressed, the less room there has been for postulating supernatural causes within a scientific account of the world, and if past experience is any guide, the trend will continue well into the future. This trend has led many to conclude that there probably are no genuine instances of supernatural causation. As science explains more of the natural world around us, appeals to supernatural causation become less plausible.
Many philosophers and scientists have concluded that the best explanation for our ability to develop successful scientific explanations for such a wide range of phenomena in terms of natural causes is that there are no genuine instances of supernatural causation. Barbara Forrest, for example, describes naturalism as "a generalization of the cumulative results of scientific inquiry" (Forrest 2000, p. 19). In other words, the best explanation for the success of science is that naturalism is true. Given the proliferation of successful scientific explanations for phenomena, Forrest concludes that there is "an asymptotic decrease in the existential possibility of the supernatural to the point at which it is wholly negligible" (Forrest 2000, p. 25). If naturalism were false, there would be some phenomena that could not be explained solely in terms of natural causes. However, because science can explain all of the uncontroversial phenomena we have encountered in terms of natural causes, there probably are no phenomena which cannot be explained in terms of natural causes. Therefore, naturalism is probably true.
This success of science argument rests on a crucial inductive premise--that we can infer that all phenomena can be explained in terms of natural causes from the ability of science to explain all of the uncontroversial phenomena we have encountered in terms of natural causes. Even if we accept the validity of this inductive inference, we still have to establish that all the uncontroversial phenomena we have encountered so far can be explained scientifically. Since there certainly are uncontroversial phenomena for which we lack successful scientific explanations--consider the prevalent gravitational influence of some unknown form of dark matter in the universe--I will defend a related but stronger argument for naturalism. This argument does not require us to have a successful scientific explanation for all well-established events in order to provide evidential support for naturalism.
A likely candidate for a supernatural event is not necessarily the result of supernatural causation given that meeting the criteria for a likely candidate is a necessary but not a sufficient condition for actually being a supernatural event. Thus, if naturalism is true, it does not necessarily follow that there will be no likely candidates for a supernatural event--it is possible, however unlikely, that a naturally-caused event would also meet the requirements for a likely candidate for a supernatural event. For example, suppose that a subject can induce out-of-body experiences at will in a laboratory setting. During several experimental trials, after this subject has induced an out-of-body experience, infrared cameras capture the outline of a person moving toward a bell which begins to ring in a room adjacent to the location of the subject's normal physical body. If such events occurred today, they would meet all of the criteria for a likely candidate for a supernatural event. Nevertheless, such events might be the result of entirely natural causes which could be understood only in terms of some future science not yet available to us. For example, one might postulate that human organisms possess natural astral bodies made of some unknown form of exotic matter which can detach from normal physical bodies in certain circumstances. In the absence of successful scientific explanations for such phenomena, however, uncontroversial instances of likely candidates for a supernatural event would make supernaturalism more likely to be true than not relative to a background scientific picture lacking natural categories for such events.
Regardless of such possibilities, if there are any events within nature that have supernatural causes, these events will be likely candidates for a supernatural event. Thus, if naturalism is false, there will be events which are likely candidates for a supernatural event. Even without a definitive set of criteria for identifying a supernatural event, we can see the beginnings of an argument for naturalism:
(P1) If naturalism is false then there are events which are likely candidates for a supernatural event.Or, to put the argument in another form:
(P2) There are no events which are likely candidates for a supernatural event.
(C) Therefore, naturalism is not false (i.e. naturalism is true).
(P1) If there are no events which are likely candidates for a supernatural event then naturalism is true.The argument above forms the basic foundation of my defense of naturalism. As stated above, it is too broad to be useful; the crucial second premise simply cannot be established in the absence of omniscience. However, we can modify this argument into a more practical lack of evidence argument:
(P2) There are no events which are likely candidates for a supernatural event.
(C) Therefore, naturalism is true.
(P1) If after an intensive search of the natural world scientists and historians have found no uncontroversial evidence for likely candidates for a supernatural event then naturalism is probably true.The lack of evidence argument assumes that if supernatural causation does occur, prima facie we should have uncontroversial evidence for events which are likely candidates for a supernatural event. There is no reason in principle why the occurrence of such events could not be established conclusively. On the other hand, if supernatural causation does not occur, we should expect to find no uncontroversial evidence for a likely candidate for a supernatural event. If naturalism is true, we will not necessarily fail to find uncontroversial evidence for a likely candidate for a supernatural event. However, we probably will not find such evidence. In other words, if we do find uncontroversial evidence for a likely candidate for a supernatural event, it is more likely than not that supernatural causation does occur and thus that naturalism is false.
(P2) After an intensive search of the natural world scientists and historians have found no uncontroversial evidence for likely candidates for a supernatural event.
(C) Therefore, naturalism is probably true.
Now that I have laid the groundwork for a defense of naturalism based on the lack of uncontroversial evidence for events which would probably have supernatural causes if they occurred, it is time to elaborate upon and defend the premises of the argument. First, since I have already used the crucial phrase without defining it, I want to clarify what I mean by 'uncontroversial evidence'. Uncontroversial evidence is not necessarily replicable experimental evidence, although that would certainly qualify as uncontroversial evidence. By uncontroversial evidence for a proposition I simply mean evidence which would lead any reasonable person to conclude that the proposition is true. For example, we have uncontroversial evidence that slavery was prevalent in 19th century America, that the continents have drifted apart over hundreds of millions of years, that the evolution of species has occurred, and that light is a form of electromagnetic radiation. What these propositions have in common is that they are accepted by a consensus of the experts doing research within the relevant empirical subject matter. Uncontroversial evidence is evidence that generates consensus among the experts in the relevant field.
One might object that science could never falsify naturalism because scientific explanations are never cast in terms of supernatural causes. However, while scientific explanations are inherently naturalistic, scientific discoveries could strongly suggest that an event has occurred which could not plausibly be explained in terms of natural causes. For example, had human beings been the only life to appear on the planet Earth immediately after it was habitable, with no evidence of evolution from previous ancestors and no fossils of extinct species ever found, this would be a scientific discovery which would strongly suggest a supernatural cause of the origin of human beings. Science has undermined the credibility of all forms of supernaturalism not because science assumes that only natural causation occurs as a methodological principle but because science has been successful in using that assumption. There simply are no gaps in our scientific picture of the world which seem to require an appeal to supernatural causes. The simplest and most straightforward explanation for the success of methodological naturalism as a scientific strategy is that metaphysical naturalism is true.